Because it provides certainty!: Cap and Tax . Is . Awesome?

Uneasy Emission Traders Seek Help From Congress on CAIR Replacement - NYTimes.com

Existing allowances of SO2 traded at about $5 per ton last week, down from about $15 per ton before the release of the transport rule. They had hovered around $300 per ton before CAIR's rejection in 2008.
...
With SO2 allowances trading at about $5 per ton, and little prospect of carrying over the permits into the new program, utilities have little incentive to bank allowances or add emissions controls for the time being, traders say. Because those controls have upkeep costs beyond the original investment, some plants might even find it more cost-effective to use allowances than to turn on scrubbers that have already been installed, traders said.